Not even close.
GT1 was late 1997/early 1998, right?
Obviously you're not familiar with the early work of David Kaemmer and Geoff Crammond. Kaemmer and Papyrus started with Indy 500 in 1989, starting a series of simulation Indy and NASCAR games. Crammond and Microprose released F1 Grand Prix in 1992, starting another important simulation series, the Grand Prix series.
So let's put aside the claim that GT1 was the first racing game to be considered a simulator. That's just false.
Gran Turismo was the first serious simulation on a console, and the first to achieve major commercial success. But go back and play something like GPL which released at almost the same time, and you'll realise how limited it's claim to simulation was. It was decent, but not great. Not entirely unlike GT6 today. It's a passable simulation but severely limited in many areas.
The thing about GT1 is that it feels remarkably similar to drive to many games that are labelled arcade or semi-sim today. It was hard to tell at the time because real simulations weren't as widespread and easily available as they are today. Today, it's easy to pick up Assetto Corsa or rF2 or iRacing and see how nuanced their physical models are over what GT provides. Way back when, it's likely that Gran Turismo was all people had or would see, and so they assumed (like you do) that it was the pinnacle of simulation.
GT1 isn't much of a simulator, because it's built along the same lines as the semi-sim and arcade games of today. It does the same trick that games like NFS: Shift, GRID, and Driveclub do. It provides a believable and authentic feeling experience without necessarily getting all anal about being accurate. I happen to enjoy those sorts of games and I think early Gran Turismos did it very well, finding a nice balance between simulation and pure fun.
But I don't fool myself by saying that GT1 was a simulation comparable with the best of the time, because it wasn't. And it certainly wasn't the first by any stretch of the imagination. Which is why I say it wasn't much of a simulator. In some senses it was, but in many senses it wasn't and it's simulation certainly wasn't it's greatest strength.